The rising incidence of breast cancer has reached epidemic proportions. In a recent National Institutes of Health consensus development conference, it was emphasized that during the 1990's, more than 1.5 million women in the U.S. will be diagnosed with breast cancer and that approximately 30% of them will die of the disease (1). It is estimated that 1 out of every 9 women will develop breast cancer during her lifetime. In general, the smaller the lesion is at the time of detection, the better the prognosis. Magnetic Resonance Imaging (MRI) offers exciting potential for increased tissue characterization compared to other imaging modalities. The major goal of this proposal is to improve the specificity of the characterization of lesions in the breast as either benign or malignant based on an analysis of their metabolism using localized magnetic resonance spectroscopy (MRS). Currently, X-ray mammography is the major modality used for screening breast cancer. For women who do not have dense breasts mammography has an overall sensitivity of about 90%, with a positive predictive value of about 20-30%. For reasons of cost and availability, it is unlikely that MRI of the breast will replace mammography as the primary screening test even though preliminary studies using MRI have found about equivalent sensitivity with variable specifities that are significantly higher than mammography. There are indications that the specificity of MRI will continue to increase through both technical advances and improvements in the ability of radiologists to interpret the images. Because of its much higher specificity it is more likely that MRI will be employed to provide improved characterization of lesions visualized on mammography or in patients with a high clinical suspicion of disease (e.g. with a palapable nodule). The development of image-guided Magnetic Resonance spectroscopy (MRS) techniques provides a means for obtaining metabolic information for lesions that can be identified on MRI. We have validated spatial localization methods for acquiring solvent-suppressed proton spectra from lesions as small as (6 mm)3 which make this modality applicable to study breast disease. Our central hypothesis is that this metabolic information alone, or in combination with MRI criteria, can be employed to improve the specificity of the test. This improvement in specificity will reduce the number of negative biopsies performed resulting in cost saving without having an adverse impact on patient care.